This invention relates to a circuit for a displacement detector incorporating with a sensor coil forming a differential transformer.
FIG. 28 shows the circuit structure of a prior art contact displacement detector with a sensor 201 incorporating a differential transformer 202 and a sensitivity-adjusting resistor 203. The differential transformer 202 has a mobile core (not shown) and two coils 204 and 205 disposed around this mobile core. These two coils 204 and 205 are connected in series and driven by an AC voltage provided as a driving signal from an oscillator 206 through an amplifier 207. Output signals are taken out from a junction point in between.
This displacement detector is a transducer of the half-bridge type. The inductance of the two coils 204 and 205 driven by an AC voltage is"" a function of the position of the mobile core. The inductive voltages generated in the two coils 204 and 205 are equal to each other if the mobile core is at the center of the two coils 204 and 205. If the mobile core is displaced from this center position, the inductive voltage of one of the coils 204 or 205 increases and that of the other coil 204 or 205 decreases. A contact member (not shown) for contacting the target object of measurement is attached to this mobile core and the sensor is adapted to detect the displacement of this contact member.
The output signal from the junction at the center of the two coils 204 and 205 is an AC output of which the amplitude changes according to the displacement of the mobile core. After being amplified by an amplifier 208, this output AC signal is subjected to a full-wave rectification process by an AC-DC converter 209 and inputted to the non-inversion input terminal of a differential amplifier 210. Another AC voltage applied from amplifier 207 to the differential transformer 202 is inputted to the inversion input terminal of this differential amplifier 210 through amplifier 211 and AC-DC converter 212 to serve as a standard signal. The differential amplifier 210 amplifies the standard signal and the output signal from the differential transformer 202 differentially and outputs a signal corresponding to the displacement of the mobile core.
According to this illustrated example, not only is the sensor 201 itself provided with a sensitivity-adjusting resistor 203, but the amplifier 208 for amplifying the output signal from the differential transformer 202 is provided with a gain-switching resistor 213 such that the gain of the amplifier 208 can be changed, depending on the kind of the sensor 201, that is, such that the same circuit can be used with sensors of different kinds with different ranges of measurement (or strokes).
According to this example, furthermore, a pull-down resistor 214 is connected to the output signal line of the differential transformer 202 and there is also provided a comparator 215 for comparing the output from the AC-DC converter 209 with a threshold value to provide a detection output. If there is a breakage in the sensor cable connected to the sensor 201, or when the wire for transmitting a signal for driving the sensor 201 is broken (as indicated by A1) or the sensor signal output line A2 is broken, for example, the AC voltage signal outputted from the sensor 201 is not communicated and becomes zero by the pull-down resistor 214 such that the breakage can be detected by the comparator 215. If the breakage is only in the grounding line, as shown by A3, the sensor driving signal is not divided by the coils 204 and 205 and hence the sensor driving signal is directly outputted. This, too, can be detected by the comparator 215.
For carrying out measurements with a high level of accuracy with such a prior art sensor, very small signals from the differential transformer must be taken out at a high level of stability and with a high S/N ratio. Moreover, the output from the amplifier 207 to become a standard signal must also be stable. For this purpose, an oscillator and an amplifier such as an operational amplifier with high accuracy and stability are required. For obtaining a high S/N ratio and stability, a dedicated IC incorporating an operational circuit for temperature compensation, etc. must be used, and this affects the production cost adversely.
Since different sensors have different sensitivities, furthermore, the gain of the amplifier 208 is adjusted by means of the gain-switching resistor 213. Thus, if a sensor with low sensitivity is used, the S/N ratio becomes lowered as the gain is increased. Although it is desirable to use processing systems having similar processing characteristics for the standard signal and the output signal from the differential transformer 202, the processing system for the output signals from the differential transformer 202 is different from that for the standard signal, being adapted to switch to change the gain. Thus, it is difficult to make the temperature characteristics of the components uniform and to place the components in a thermally well balanced manner.
Moreover, since the breakage of the sensor cable is detected on the basis of the output AC signal, if the inductance of the differential transformer 202 is increased in order to improve the sensitivity of the sensor 201, the output AC signal from the differential transformer becomes unstable due to the capacitive coupling between the signal lines at both ends of the coil 204 or 205 when there is a breakage in the sensor cable and the breakage may not be detected dependably. It may be attempted therefore to reduce the resistance of the pull-down resistor 214 in order to reduce the effect of the capacitive coupling but if the resistance of the pull-down resistor 214 is reduced, the linearity characteristic of the differential transformer 202 becomes adversely affected. A similar result is obtained even if a pull-up resistor is used instead of the pull-down resistor.
Another problem of prior art displacement sensors of this kind relates to their structure. If the diameter of a sensor is reduced from xcfx868 to xcfx866, for example, the sensor can be attached to a target object (such as a machine) more intimately and the target object can be made more compact. Since the weight of the mobile parts of the sensor must be reduced accordingly, the load to the sensor can be reduced and hence the sensor becomes usable for the measurement of an object which could not be measured because of its large load. When the diameter of a sensor is reduced from xcfx868 to xcfx866, however, it is not sufficient to merely reduce its linear dimensions to three quarters (0.6/0.8) of the original and to reduce the cross-sectional area by a factor of (0.6/0.8)2=0.56. It cannot be ignored that stoppers for the rotation of a mobile component for driving the core member, for example, must retain their original function and capability. Moreover, the difficulty in assembly because of reduced size of components must be considered and the need for water-proofing between the mobile components for the core member becomes more important.
FIG. 23 shows the structure of an example of prior art displacement sensor, having a linear bush 81 and the bobbin assembly of a differential transformer 95 inside a housing 80. A mobile member 101 having a mobile shaft 91 and a core member 89 connected to this mobile shaft 91 is movable longitudinally inside this housing 80 through a linear bush 94. The core material 89 is inserted into the bobbin assembly of the differential transformer 95 to form the differential transformer 95. The mobile member 101 is biased by means of a spring member (not shown) such as a parallel coil spring with invariable coil diameter so as to, protrude the tip of the mobile shaft 91 out of the housing 80 and a contact member 93 is formed at the protruding portion of the mobile shaft 91. The linear bush 94 is of a structure having inserted inside an outer tubular body 81 with an elongated hole 88 on its circumference a ball guide 84 with many balls 84a held on its circumference. A rotation-preventing pin 92 on the shaft 91 is inserted into the elongated hole 88 of the outer tubular body 81 so as to stop the rotation of the mobile member 101.
FIG. 24A shows another prior art displacement sensor, having a linear bush 94 and the bobbin assembly of a differential transformer 95 inside a housing 80. A mobile member 101 having a mobile shaft 91 and a core member 89 connected to this mobile shaft 91 is movable longitudinally inside this housing 80 through the linear bush 94. The core material 89 is inserted into the bobbin assembly of the differential transformer 95 to form the differential transformer 95. The mobile member 101 is biased by means of a spring member (not shown) such as a parallel coil spring with invariable coil diameter so as to protrude the tip of the mobile shaft 91 out of the housing 80 and a contact member 93 is formed at the protruding portion of the mobile shaft 91. A rotation-preventing member 102 on the mobile shaft 91 has a groove 102A formed extending in the axial direction of the housing 80. A metallic rotation-preventing guide pin 103 is pressed into a hole 80a in the housing 80 and into the groove 102A as shown in FIG. 24B to prevent the rotation of the mobile member 101.
FIG. 25 shows the structure for leading a cable out of the housing 80 in a sealed manner, including a cable-stopping member 110 having a resin-filling portion 108 and a cable-passing opening part 109. After a cable 96 is inserted into the opening part 109, an O-ring 111 is placed between the resin-filling portion 108 and the opening part 109, and the resin-filling portion 108 is filled with an epoxy resin material 112, and the cable-stopping member 110 is pressed into the backward end of the housing 80.
With a prior art displacement sensor structured as shown in FIG. 23, the rotation-preventing pin 92 is inserted into the elongated hole 88 on the outer tubular body 81 of the linear bush 94 in order to prevent the rotation of the mobile member 101. Thus, the elongated hole 88 must be formed on the outer tubular body 81 and burrs are left on the inner surface of the outer tubular body rubbed by the balls 84a. A work process for removing these burrs becomes necessary, and the construction and preparation of this linear bush becomes complicated. Such means for preventing the mobile member from rotating cannot be used directly with a displacement sensor as the latter is becoming miniaturized because the workability efficiency will be significantly affected. In particular, the distance of the displacement in the direction of the motion of the mobility becomes increased and the outer tubular body 81 of the linear bush 94 comes close to the differential transformer 95. Thus, the magnetic property of the outer tubular body 81 may come to adversely affect the characteristics of the differential transformer 95, causing the product quality and temperature characteristic to become unstable.
With a prior art displacement sensor structured as shown in FIG. 24A adapted to have a metallic guide pin 103 inserted into a groove, the pin 103 will slide inside the groove 102A as the mobile member 101 is moved, and this increases the friction between metals, making it difficult to reduce the force required for the operation. Since the housing 80 has a hole 80a for accepting the pin 103, water-proofing cannot be made and the device diameter cannot be reduced because the portion around the hole 80a must be made sufficiently thick. Additional problems are that the length in the mobile direction increases and the production cost of the housing 80 becomes higher. Since a parallel coil spring with invariable coil diameter is used for the mobile shaft 91, furthermore, the coil will rub against the neighboring components to cause friction and interference. Moreover, since the cable is fastened to the housing 80 by passing the cable 96 through the opening part 109 of the cable-stopping member 110, placing the O-ring 111 between the resin-filling portion 108 and the cable-passing opening part 109, filling the resin-filling portion 108 with the epoxy resin 112 and pressing the cable-stopping member 110 into the back end of the housing 80, there is a large variation in the strength and the cable cannot be kept flexible.
FIG. 26 shows still another prior art displacement sensor providing a housing 80-1 with a female screw part 120 and an outer tubular body 81-1 of a linear bush 94 with a male screw part 121, It is assembled with the male screw part 120 engaged with the female screw part 121 to tighten a flat packing member 122 in a watertight manner. Since it requires a height corresponding to the ridge portion of the male screw part 121, the sensor is prevented from being made compact. An adhesive may be used instead of screws, but this leaves the problem of dependability in the sealing.
FIG. 27 shows still another prior art displacement sensor assembled by inserting a rubber boot 123 onto a mobile shaft 125 from the side of a measurement piece 124. Since the mobile shaft 125 is provided with a male screw part 126 for attaching the measurement piece 124, the inner surface (sealing surface) of the sealing part 123a of the rubber boot 123 is easily damaged, and this again leaves the problem of dependability in the sealing.
It is therefore an object of this invention to provide a circuit for a displacement detector capable of detecting displacements accurately.
It is another object of this invention to provide a circuit for such a displacement detector which can be produced inexpensively.
It is still another object of this invention to provide a circuit for a displacement detector capable of dependably detect a wire breakage.
It is a further object of this invention to provide a contact displacement sensor structured so as to be made compact while maintaining its original functions and capabilities.
Since the amplitude of the driving signal for driving the differential transformer is adjusted according to the kind of the differential transformer, or the sensitivity of the differential transformer, the S/N ratio does not drop as in the case of a prior art detector adapted to adjust the gain. Since the gain is not switched, the signal processing by the standard signal processing means and the output signal processing means can be made equal. As a result, the components can be arranged in a thermally balanced manner.
Since the amplitude adjusting means adjusts the amplitude of the driving signal such that it will take upon a value corresponding to the aforementioned standard value, the standard signal and the standard value become nearly equal, and similar merits as described above can results if the standard value instead of the standard signal is given to the differential amplifier means.
According to another embodiment of the invention, the standard signal processing means and the output signal processing means each comprise an amplifier circuit and an AC-DC converter, and at least either these amplifiers or these AC-DC converters are thermally coupled, for example, by being packaged together or by being placed appropriately. Thus, not only temperature variations in the standard signal processing means are automatically corrected according to this invention because the standard signal is fed back, temperature variations also become alike in the standard signal processing means and the output signal processing means because they are thermally coupled. Their variations can be cancelled together by the differential amplifier means on the downstream side, and a highly accurate detection becomes possible.
According to still another embodiment of the invention, an abnormal condition of the detector is detected on the basis of the level of a DC bias which is superimposed to the output signal from the differential transformer. In the case of a breakage in the sensor cable, the level of the DC bias superimposed to the output signal from the differential transformer becomes outside a specified range, and this makes it possible to detect a breakage in the cable. Even if the inductance of the differential transformer is high, such an abnormality can be reliably detected without being affected by the thermal coupling, and there is no need to reduce the resistance of a pull-down resistor or a pull-up resistor. A trouble in the driver means can also be detected similarly.
A displacement sensor embodying this invention may be characterized as comprising a linear bush and a mobile member inside a housing and rotation-preventing means for preventing rotation of the mobile member. The linear bush includes an outer tubular body extending in its axial direction and containing a holder which is movable in the axial direction of the outer tubular body. The mobile member has a mobile shaft supporting the core member of a differential transformer and is movable in the same axial direction, being biased outwardly by a spring. The outer tubular body of the linear bush and the holder inside the outer tubular body are each provided with a hole, and the mobile shaft of the mobile member includes a pin-accepting hole part. The rotation-preventing means comprises a rotation-preventing member such as a pin which is inserted movably through these holes in the outer tubular member and the holder and into the pin-accepting hole part. The outer tubular body of the linear bush is a tubular member to be attached to the inner surface of the housing when the linear bush is engaged with the housing and adhesively attached to the housing. The holder may comprise a ball guide holding many balls thereon.
With a structure as described above, the rotation-preventing means can be contained inside the main body of the sensor such that the sensor can be made compact and shorter. It also helps to increase the strength of the mobile shaft and its production becomes easier.
In another aspect of the invention, the housing has protrusions formed thereon, protruding in the inward direction towards its interior, each protrusion has a stopping surface perpendicular to the axial direction of the sensor, and the housing includes a stopper having an outer surface with flat parts and a contact surface which is at one end of these flat parts and is also perpendicular to the axial direction. These protrusions are positioned at the flat parts around the stopper and the contact surface and the stopping surface contact each other to position the stopper and to prevent the stopper from rotating. With the sensor thus structured, the stopper can be affixed to the housing, positioned and prevented from rotating as the stopper is inserted inside the housing with the protrusions positioned at the flat parts and the stopping surface and the contact surfacing contacting each other.
These protrusions are produced according to this invention by punching the housing inward by the so-called xe2x80x9cpunch-stretch forming methodxe2x80x9d and grinding its outer surface areas in a centerless grinding process to reduce the thickness of the housing while maintaining a specified amount of protrusion. By such a method, protrusions with a specified height can be produced even if the material of the housing is relatively thick and since the housing is made thinner, the sensor can be made more compact accordingly.
The displacement sensor embodying this invention may be so structured that both the housing and the mobile member have two (first and second) stopper parts for preventing the aforementioned rotation-preventing member from hitting a near-by component and becoming thereby deformed. The stopper parts are so positioned that as the spring is stretched as much as possible until the first stopper parts come to contact each other (the spring being at the xe2x80x9cstretched limit positionxe2x80x9d), there is a finite interval between the rotation-preventing member and one of the end parts of the holes into which the rotation-preventing member is inserted and that as the spring is pushed in and contracted as much as possible until the two second stopper parts come to contact each other (the spring being then at the xe2x80x9cpushed-in positionxe2x80x9d), there is similarly another finite interval between the rotation-preventing member and the other of the end parts of the same holes. These two first stopper parts may be formed respectively on the outer tubular body of the linear bush and the stopper, and the first and second stopper parts of the mobile member may be formed on a core shaft. With stopper parts thus formed, the rotation-preventing means does not hit either the front or back end part of the elongated hole in which it slides as the mobile member is moved by stretching or contracting the spring in either direction. Thus, the rotation-preventing means is not deformed.
According to a preferred embodiment, a conic coil spring is used, supported between the stopper and the core shaft and the core shaft includes a tapered part for avoiding interference with this conic spring. Thus structured, the conic spring is not interfered by neighboring components even if the sensor is made compact as a whole.
The mobile shaft and a holder for the contact member (the xe2x80x9ccontact-member holderxe2x80x9d) may be realized as separate components, and a rubber boot is attached by engaging its front and back end parts respectively with a front boot holder on the contact-member holder and a back boot holder on an end cap with which the outer tubular member is provided. The contact-member holder is thus attached to the rubber boot, and the contact-member holder is connected to the rubber boot. In this manner, the inner surface of the rubber boot is not damaged and the end cap makes the structure even more reliably watertight.
Where a cable is connected to the sensor, a cable cap of a synthetic resin material is integrally formed with the cable and engaged with a back end part of the housing such that the cable is pulled out of the back end part of the housing. Polyester elastomer may be used for this purpose. Since the cable-holding part thus formed is not a separate component of the sensor, the total number of the constituent parts is reduced and the production cost can also be reduced. Furthermore, the cable can be made more flexible.
In order to attach the cable cap to the housing in a watertight manner, a groove is formed on the cable, filled with an adhesive. A protrusion is formed in the groove such that it will contact the inner surface of the housing as the cable cap is attached to the housing and the adhesive is sealed inside.